In recent years, there has been a demand for synthetic polymers in fine particle form, as a property-modifier for plastics, fiber, film, etc. and as a surface-processing material for paper, film, leather, etc. Among others, it is demanded to use AN polymers in fine particle form in the above-mentioned fields because AN polymers are excellent in resistance to light, resistance to weathering and resistance to solvents.
As a process for producing such fine particles of AN polymers, we have, for example proposed an AN polymer aqueous emulsion in Japanese Patent Publication No. 2207/1980. This emulsion is useful as aqueous dispersion, but in order to obtain a dispersion in an organic solvent, it is not only necessary to once separate the polymer from the aqueous emulsion and then disperse it again in the solvent, but also it is very difficult to remove the polymer in fine particle form from the aqueous emulsion because such operation will cause agglomeration and agglutination of the polymer particles.
Accordingly, it is desired to polymerize an AN monomer directly in an organic solvent. However, the cohesive force between AN polymer particles is generally strong, so that when polymerization is carried out in the usual way, the generated polymer particles unite with one another to form coarse, bulky agglomerated particles. Such agglomerated particles can no longer be separated into individual particles by any mechanical means. Therefore, it has been very difficult to obtain fine and uniform AN polymer particles dispersed in an organic solvent.
To overcome this difficulty, in Japanese Patent Publication No. 34396/1970, for example, there has been proposed a method preventing the agglomeration and agglutination of AN polymer particles. In that process an oil-soluble high molecular substance is added as a dispersing agent to increase the viscosity of the polymerization system. However, the effect of preventing agglomeration and agglutination by such a means is not sufficient. Moreover, since there is a need for adding a large amount of the oil-soluble substance, this method undergoes a considerable restriction in practical use. In Japanese Patent Publication No. 8127/1976, a method is proposed in which a monohydric alcohol of 1 to 4 carbon atoms is used as an organic solvent. In this method, not only the kind of solvent used is limited but also the effect of uniform dispersion and stability is insufficient. In this publication it is described that the particles sediment to the bottom of the container during storage.